Wind energy installations are generally connected to a power supply system for transmitting electrical power via a transformer. The transformer serves the purpose of raising the comparatively low voltage level present at the wind energy installation itself to a higher voltage level (medium voltage or high voltage) of the power supply system. Generally, wind energy installations produce the electrical power at a voltage level of up to 1 kV (a frequently used voltage is 690 volts), and these wind energy installations are connected to a medium-voltage power supply system (voltage range approximately 20 kV) by means of the transformer; some more recent wind energy installations with a high power, such as the REpower 5M or 6M can provide the power output thereby at a medium voltage level as well, with the transformer being located within the wind energy installation. The transformers transmit all of the power generated by the wind energy installation and are therefore subjected to a high load, in particular in the case of relatively high load states. For protection against over-loading, it is therefore known to provide a monitoring device for the transformer. It is known to provide a voltage measurement on one side of the transformer. This voltage measurement is used to determine whether the voltage is in a predetermined range in order thus to isolate the wind energy installation from the power supply system by way of precaution in the event of said voltage departing from the predetermined range.
In wind energy installations with powers in the range of up to 2 MW, it is conventional that the voltage on the low-voltage side of the transformer, i.e. on the side of the wind energy installation, is monitored. This arrangement provides the advantage of effective protection of the wind energy installation and of the transformer. However, it has the disadvantage that depending on the load case as a result of the transformer voltage differences of up to 5% may arise in comparison with the theoretical transformation ratio. There is therefore the risk of excessive voltage discrepancies occurring on the high-voltage side of the transformer, i.e. of the desired range not being adhered to. In other more powerful wind energy installations, the opposite concept is often used. This consists in the voltage on the high-voltage side of the transformer, i.e. on the system side, being monitored. This arrangement provides the advantage of improved monitoring of the response of the wind energy installation on the power supply system to be protected. However, in this case too, the situation occurs in which, via the transformer, impermissible voltage discrepancies can arise on the other side, in this case the low-voltage side, which amount to more than 5%. In the event that the measuring device is arranged on the high-voltage side of the transformer, there is therefore the risk of the limit values not being adhered to on the low-voltage side. In order to protect the wind energy installation, in this case possibly special measures are required, such as monitoring to ensure that a minimum active power is adhered to (at the expense of a reactive power output which is desired per se) for ensuring a sufficient internal power supply to the wind energy installation (DE-A-103 44 392).
There is in principle the possibility of compensating for the variations in the voltage which are induced by the transformer by selecting the tolerance zone to be correspondingly narrower, but this has the disadvantage that the possible working points for the wind energy installation are therefore (considerably) restricted, which results in a corresponding reduction in the energy yield. The selection of a narrower tolerance range therefore results in a reduction in the efficiency and the block diagram therefore in noticeable losses in terms of profitability.